The present invention relates to a digital audio broadcast receiver, more particularly to the method by which a digital audio broadcast receiver acquires frame synchronization.
It will be assumed that the received digital audio broadcast signal, referred to below as a DAB signal, complies with Recommendation BS.774 of the Radiotelecommunication Sector of the International Telecommunications Union (ITU-R), entitled xe2x80x9cService requirements for digital sound broadcasting to vehicular, portable, and fixed receivers using terrestrial transmitters in the VHF/UHF bands.xe2x80x9d The broadcast signal is accordingly divided into frames, each beginning with a null symbol in which the carrier amplitude is reduced to zero as a frame synchronization signal.
In the rest of each frame, orthogonal frequency-division multiplexing (OFDM) is used to combine a plurality of subcarrier signals onto which digital data are modulated by differential quaternary phase-shift keying (QPSK). Powerful error-correcting techniques, including interleaving and convolutional coding, enable the digital data to be transmitted at high speed with high reliability, even to mobile receiving stations experiencing substantial multipath fading. The digital data comprise compressed audio data coded according to the ISO/MPEG Audio Layer Two standard.
Incidentally, ISO stands for International Standards Organization, and MPEG for Motion Picture Experts Group.
A digital audio broadcast receiver acquires frame synchronization by detecting the null symbols at the beginning of each frame. The receiver must contend with four BS.774 transmission modes, having three different frame lengths and four different null symbol lengths. The receiver must infer the transmission mode from the frame and symbol lengths. The receiver must also contend with momentary fading and other types of noise, which may be falsely recognized as frame synchronization signals.
A conventional method of acquiring frame synchronization, which will be described in more detail later, starts by detecting the interval between frame synchronization signals (null symbols), using a gate circuit to block noise occurring at times when no frame synchronization signal is expected. When frame synchronization signals have been observed at a sufficient number of regular, consecutive intervals equal to the frame length in one of the transmission modes, it can be assumed with a high degree of probability that the observed frame synchronization signals are valid signals, not caused by noise. Next, if necessary, the length of the frame synchronization signals is detected to discriminate between transmission modes having the same frame length but different symbol lengths.
One problem with this method is that if a noise pulse is incorrectly recognized as a frame synchronization signal, the gate circuit may operate at the wrong times, blocking valid frame synchronization signals. A period at least equal to the longest frame length then elapses before the mistake is recognized. When the mistake is recognized, the search for frame synchronization signals must begin anew.
Another problem is that discrimination between the two transmission modes having equal frame lengths does not begin until the frame length has been identified. Reliable discrimination requires the measurement of the lengths of a number of frame synchronization signals, so the entire process is time-consuming.
A further problem is that the gate circuit does not block noise pulses occurring near expected frame synchronization signals. When a frame synchronization signal is immediately preceded by a noise pulse, for example, the length of the noise pulse may be measured instead of the length of the frame synchronization signal, leading to incorrect mode discrimination.
An object of the present invention is to acquire frame synchronization in a digital audio broadcast receiver quickly and reliably, despite the presence of noise.
The invented digital audio broadcast receiver has a synchronization signal detector for detecting frame synchronization signals, a control unit for acquiring frame synchronization according to the detected frame synchronization signals, a timer for measuring pulse widths and intervals, and a memory. The memory stores a history of the pulse widths of the detected frame synchronization signals, of the intervals between these signals, and of counts of frame synchronization signals of predetermined pulse widths detected at predetermined intervals.
By maintaining a history of past pulse widths, intervals, and counts in the memory, the control unit is able to consider both pulse widths and intervals from the beginning of the acquisition process, and to recover from mistakes made due to noise without having to start counting over again from zero.